Side well-charged reverberatory aluminum melting furnaces were developed to provide improved aluminum recovery (reduced aluminum melt loss) as compared to direct-charged, direct-fired reverberatory aluminum melting furnaces. Aluminum melt loss, or undesired oxidation of aluminum during the melting process, is a large cost to the industry, and can range from roughly 0.5% to as much as 5% of the incoming scrap charge, depending on the type of scrap and the melting process. (Das, Subodh K. “Reduction of Oxidative Melt Loss of Aluminum and its Alloys”, DOE Report #DE-FC36-OOID13898, February 2006)
Over the years, various melting processes, techniques, burner types and practices have been developed with a focus on minimizing aluminum melt loss, however melt loss remains a significant cost to the industry, and there is room for further improvement.
In the direct-fired furnace, aluminum charge materials are placed directly into the main hearth, where the burners (typically natural gas, fuel oil or other fossil fuel) fire directly onto the charge pile. In these furnaces, melting is a batch process, and there can be direct flame impingement onto the charge materials.
With lighter gauge (thinner section) scrap types, with direct flame impingement typically melt loss (aluminum oxidation) is increased. With thicker sections of aluminum scrap, such as sows, T-bars, ingot crops or larger castings, due to the high thermal conductivity of solid aluminum, heat from direct flame impingement is quickly conducted through the aluminum. Due to the relatively low surface area to mass (volume) ratio, surface temperatures do not rise to excessive levels, as the heat can be conducted into the interior of the aluminum mass. The temperature of the large solid scrap piece essentially rises uniformly. However, with thinner pieces of aluminum scrap, with higher surface area to mass (volume) ratios, with direct flame impingement surface temperatures can rise more quickly, to much higher levels, as there is less mass to conduct away heat applied to the surfaces. Surfaces can start to melt, and oxidize, more quickly, especially with direct flame impingement. This phenomenon defines a theoretical critical scrap thickness. Below this critical scrap thickness, melt loss can be increased significantly. (Van Linden, Jan and Vild, Chris “New Melt Technology for Aluminum Recycling”, Proceedings from the 7th International Extrusion Technology Seminar ET 2000, page 143)
For this reason, side well-charged aluminum reverberatory melt furnaces were developed. Scrap pieces with higher surface area to mass ratios, such as sheet punchings, castings gates, risers and returns, strip and lighter gauge castings are charged into the side well, away from direct flame or flue gas contact. In the side charge well, the scrap is quickly submerged. In this manner contact with ambient air is also minimized. Molten metal pumps are commonly employed, to circulate molten aluminum between the main hearth and the side well. This molten metal circulation greatly improves heat transfer and melting thermal efficiency, and also greatly improves homogeneity of metal chemistry and temperature.
The side well-charged aluminum reverberatory melt furnace also makes it possible to have more of a continuous charge/melt operation, as opposed to strictly batch melting.
In these side well-charged melt furnaces, it is advantageous to maintain the molten aluminum surfaces very still and quiescent, with minimum agitation or turbulence. A relatively thin dross (aluminum oxide) layer forms on top of the molten aluminum surface, whether exposed to ambient air (side wells) or the burner products of combustion (main hearth). This thin dross layer acts as a protective barrier, to retard further aluminum oxidation. Whenever this dross layer is broken, by any surface agitation or turbulence or “surface rippling” effect, then more fresh molten aluminum is exposed to the atmosphere, and aluminum oxidation is increased. In order to maintain a quiet, undisturbed flat bath surface, the molten aluminum circulation (pumping) is accomplished underneath the molten surface. Molten aluminum circulates between the side wells and main hearth via “submerged arches”, passageways below the molten aluminum surface level, built into the barrier wall that separates the main hearth from the side wells.
While the side well-charged furnace has been shown to increase yield (reduce melt loss) for many types of thinner section, lighter gauge scrap, for very thin section types of scrap such as machine chips or UBC (used beverage can) shreds, the special vortex charge well was developed to further improve yield (see FIG. 1). Very thin-section aluminum scrap pieces, such as machine chips or shreds, will often float on the surface of the charge well, when charged into a conventional side well furnace. Since they are not readily submerged, while floating on the molten surface they remain exposed to the ambient atmosphere, where melt loss (oxidation) can occur as they heat up. Heat transfer efficiency can also be greatly improved if these chips/shreds could be more rapidly submerged. In some situations, mechanical “puddlers” are employed to periodically push these floating light gauge scrap pieces underneath the charge well surface. However, these mechanical devices can increase melt loss, since the molten aluminum surface and protective dross layer is agitated, exposing more fresh molten aluminum to the atmosphere.
The vortex charge well concept is shown in FIGS. 1, 2, and 4. A specially designed, bowl-shaped chamber 101 is placed between the pump well 102 and charge well 103, as shown in FIG. 1. In this chamber 101, the molten aluminum travels in a swirling pattern, creating a concave “vortex” or “toilet bowl” effect. When light gauge machine chips or shreds 110 are charged into this V-shaped molten aluminum vortex, they are very rapidly pulled under the surface. This reduces aluminum oxidation by keeping the chips/shreds 110 away from ambient air contact, and it also improves heat transfer efficiency.
While these vortex charge wells improve aluminum recovery (reduce aluminum oxidation) by more quickly and effectively submerging the light gauge solid aluminum charge pieces 110, they also contribute to some additional aluminum oxidation by virtue of their configuration. In order to create the molten aluminum vortex shape, molten aluminum is continuously exposed and re-exposed to the ambient atmosphere. The relatively flat, quiet and undisturbed molten aluminum surface is now agitated, in the vortex area, and fresh molten aluminum is continually brought to the surface and exposed to ambient air. In some cases, one can see an aluminum oxide crust or skin continually forming, breaking up, and pieces of oxide repeatedly being pulled under the vortex. These aluminum oxides then float to the top in the adjoining charge well or “float out well” 103, since this side charge well is still and with an undisturbed flat bath surface. This additional aluminum oxide is periodically skimmed out of the charge well (float out well), increasing the total dross skimming requirement.